1. Field of the Invention
The present invention relates to oil well optical apparatus. The present invention more particularly relates to optical apparatus for generating and detecting fluorescence in oil flowing in a well.
2. State of the Art
The use of optical systems for the analysis of fluids is well known. For example, as set forth in the patents incorporated by reference above, optical probes can be used downhole for measuring oil, water, and gas holdup in three-phase flows. In particular, light of excitation is coupled to a small optical probe that is deployed into a sample flow. Depending on the optical properties of the fluid surrounding the probe, the returning signal carries the optical signature of the fluid. Gas will induce a large reflectance, compared with liquids, due to the large mismatch of the index of refraction. Crude oils, on the other hand, will produce fluorescence under illumination. By analyzing both the reflectance and the fluorescence signals, the nature of the fluid in contact with the probe can be identified.
While the previously incorporated patents represent a major step forward in downhole analysis of fluids, the apparatus described therein are not as robust in certain circumstances as might be desired. For example, when borehole temperatures reach 200xc2x0 C., many semiconductor lasers stop working. The low efficiency of energy conversion and light coupling of incandescent light sources makes them difficult to use. In addition, it is very difficult to detect the fluorescence of oil in the well because the fluorescence yield (i.e., the ratio of fluorescence signal to the corresponding excitation power) of crude oils is extremely low. For source excitation at 470 nm, the fluorescence yield is typically around 10e-4 for medium density crude oils. Further, the ratio of the received fluorescence signal to the received reflected signal is very small; i.e., the reflected signal can overwhelm the fluorescence signal. Moreover, where the optical system is to be located downhole, the tight space available permits only compact optics.
It is therefore an object of the invention to provide a downhole apparatus that enables a simultaneous detection of fluorescence and reflectance from a single fiber probe.
It is another object of the invention to provide apparatus that are reliable at high temperatures and that permit detection of oil fluorescence downhole.
It is a further object of the invention to provide downhole apparatus that maximize signal sensitivity and reduce noise in the detection of oil fluorescence.
In accord with the objects of the invention, the optical apparatus of the invention includes, among other things, a light emitting diode (LED) light source, a reflectance detector, a fluorescence detector, first, second, and third optics, a dichroic mirror, a beam splitter/coupler, a probe, a short pass filter, a dichroic long pass filter, and a lens. Source light from the LED is filtered by the short pass filter and fed to the dichroic mirror which deflects light of desired wavelength and passes light of undesired longer wavelengths. The deflected light is focused by the lens onto the first optic which is coupled to a second optic by the splitter coupler. The second optic is coupled to the probe which is placed in and injects light into the fluid flow. Light reflected by the fluid flow or fluorescing therefrom is received by the probe, and provided to the splitter/coupler by the second optic. The splitter/coupler forwards a first portion of the light to the reflectance detector via the third fiber, and a second portion of the light via the first fiber to the lens. Light received by the lens is directed to the dichroic mirror which deflects reflected light (i.e., light at the LED wavelength), and passes light at longer wavelengths (i.e., wavelengths generated by fluorescence). The passed light is further filtered by the dichroic mirror to eliminate remnants of the reflected light, and provided to the fluorescence detector.
According to a preferred aspect of the invention, the beam splitter/coupler is preferably arranged to pass at least ninety percent of light received from the second fiber optic to the first fiber optic for fluorescence detection. According to another preferred aspect of the invention, the fiber optic splitter/coupler of the invention includes on a first side, a large core fiber and a plurality of small core fibers potted with a black epoxy in a metallic ferrule, and on a second side, a single large core fiber centered in the ferrule. Surfaces of both sides are polished, and an optical couplant is used to minimize insertion loss. The ferrule mechanically holds the sides together without use of a glue.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.